In current TDM-PON system, FP-LD (Fibry-Perot laser diode, also called Fibry-Perot semiconductor laser) is extensively used as a transmitter of an optical network unit (ONU), due to low cost, easy operation and simple manufacturing process, as shown in FIG. 1. FIG. 1 shows an optical line terminal 11 (OLT, which a local device), a remote node 12 (RN) and an optical network unit 13 (ONU, which is terminal device). Wherein, the optical line terminal 11 comprises a downlink transmitter 111, an uplink receiver 113, and an arrayed waveguide grating (AWG) 112. The remote node includes optical splitter 121. The optical network unit 33 comprises an arrayed waveguide grating 131, a downlink receiver 132 and an uplink transmitter 133.
However, the multimode characteristic of FP-LD limits its application. Generally speaking, the modulation rate of FP-LD is less than or equal to 2.5 Gb/s, and the transmission distance is less than or equal to 10 km. Therefore, for 10G TDM-PON system, or the transmission distance is more than 10 km, FP-LD is not suitable. Thus, a high-performance and high-cost DFB-LD (distributed feedback laser diode) laser is required to directly modulate downlink or uplink data at OLT end or at ONU end. In order to reduce the cost of the TDM-PON system, it is desirable to use low-cost FP-LD to realize high modulation rate and longer transmission distance like DFB-LD laser.
External optical injection is an effective method to improve FP-LD transmission characteristic, such as improving modulation bandwidth, reducing nonlinear distortion, reducing mode partition noise, reducing chirp, and outputting a single-mode optical wave with constant power and high side mode suppression ratio.
FIG. 2 shows an external injection locking mode of the FP-LD 21. The injected laser forces the multimode FP-LD 21 to operate in a quasi single mode, and suppresses mode partition noise. In this way, the external optical signal acts as a seed light 23 to oscillate in a FP cavity via a circulator 21, therefore the nearest mode to the peak wavelength of the injected signal will be locked to the injected light, and other modes will be suppressed. Finally, the FP-LD 21 can produce a single longitudinal mode (SLM) output with a constant power, which has nearly the same transmission performance as DFB-LD.
However, the solution has the following disadvantages:
1. if the scheme is applied to an ONU, external injection locking mode requires an additional light source at each ONU, thus adding system cost.
2. in the existing system, external injection light source is considered as an independent device, thus it is not easy to integrate the device into an ONU module unit.
3. the external seed light source is an active component, and it needs to control its on/off state.
Therefore, the solution is not feasible that employing external injection locking mode to improve FP-LD characteristic to achieve a better transmission feature in an actually realized TDM-PON system.
In addition, as a large number of new advanced multimedia applications appear, such as the service deployment of 3D TV, remote medical services, online games, interactive video electronic learning etc., there has been a great increase in the need of the network bandwidth bearing these applications. NG-PON2 (Next Generation Passive Optical Network) becomes a hot topic in ITU-T (Telecommunication Standardization Sector of the International Telecommunications Union) and FASN (Full Service Access Network). Most operators expect NG-PON2 to provide more bandwidth, higher optical divide ratio, longer transmission distance and greater access capability. Currently, both of FSAN and ITU-T determine their need of NG-PON2, to improve the available bandwidth to a rate of up to 40 Gb/s.
Among all the candidate technical solutions, TWDM-PON (Time Wavelength Division Multiplexing) has been considered as a primary solution for NG-PON2 in recent FSAN meeting, wherein 4 XG-PONs (that's 40G PON) are stacked by four 10G GPON, and the typical optical divide ratio is 1:64, thereby achieving an aggregate rate of 40 Gbps in downstream and 10 Gbps in upstream. In a single wavelength, TWDM-PON reuses XG-PON (that's 10G PON) downstream multiplexing and upstream access technology, timeslot granularity, multicast capability, and bandwidth allocation mechanism.
In TWDM-PON access, an ONU transmitter must be able to adjust any of four upstream wavelengths. Thus, it is need to design a low-cost ONU transmitter with tunable wavelength to reduce the cost of 40G TWDM-PON.
Tunable wavelength technology is an effective scheme to realize an tunable ONU transmitter, wherein, the tunable laser is used in an uplink signal transmitter, shown as FIG. 3. FIG. 3 shows an optical line terminal 31, a remote node 32 and an optical network unit 33. Wherein, the optical line terminal 31 comprises transmitters 3111-3114, uplink receivers 3131-3134, arrayed waveguide grating 312 and 315, and circulator 314. The remote node includes an optical splitter 321. The optical network unit 33 comprises an arrayed waveguide grating 331, a tunable filter 3321, a receiver 3322 and a tunable laser 333.
However, the tunable laser is a very expensive device. Especially, NG-PON2 needs to support the optical divide ratio of more than or equal to 1:64, and this means at least 64 tunable lasers are needed in a ONU transmitter, which will lead to an enormous cost for realization, and thus inevitably limits the large scale deployment of TWDM-PON system.